When Vladimir Putin sent troops into Ukraine in late February, the Russian president vowed his forces would not occupy the neighbouring country.

But as the invasion reached its 100th day on Friday, Russia seemed increasingly unlikely to relinquish the territory it has taken in the war.

The rouble, now an official currency in the southern Kherson region, is set to replace the Ukrainian hryvnia.

Residents there and in Russia-controlled parts of the Zaporizhzhia region are getting offered Russian passports.

The Kremlin-installed administrations in both regions have talked about plans to become part of Russia.

The Moscow-backed leaders of separatist areas in eastern Ukraine's Donbas region, which is mostly Russian-speaking, have shared similar intentions.

Mr Putin recognised the separatists' self-proclaimed republics as independent states two days before launching the invasion.

Fighting has intensified in Ukraine's east as Russia seeks to "liberate" all of the Donbas.

The Kremlin has largely kept mum about its plans for the cities, towns and villages it has bombarded with missiles, encircled and finally captured.

Spokesman Dmitry Peskov said it was up to people living in seized areas to decide where and how they want to live.

Annexing more land from Ukraine was never the main goal of the invasion, but Moscow is unlikely to let go of its military gains, according to political analysts.

"Of course (Russia) intends to stay," Andrei Kolesnikov, senior fellow at the Carnegie Endowment for International Peace, said.

To Russia, "it's a pity to give away what has been occupied, even if it was not part of the original plan".

Mr Putin has described the goals of the invasion somewhat vaguely, saying it was aimed at the "demilitarisation" and "denazification" of Ukraine.

It was widely believed that the Kremlin intended initially to install a pro-Moscow government in Kyiv and to prevent Ukraine from joining Nato and taking other steps away from Russia's sphere of influence.

Russia captured much of Kherson and neighbouring Zaporizhzhia early in the war, gaining control over most of Ukraine's Sea of Azov coast and securing a partial land corridor to the Crimean Peninsula, which Russia annexed from Ukraine in 2014.

There was hardly a warm welcome from the locals.

Residents of the cities of Kherson and Melitopol took to the streets to protest against the occupation, facing off with Russian soldiers in plazas.

Ukrainian officials warned that Russia might stage a referendum in Kherson to declare the region an independent state.

No such referendum has taken place, although the Russians appeared determined to hold on to both regions.

They installed people with pro-Kremlin views to replace mayors and other local leaders who had disappeared in what Ukrainian officials and media said were kidnappings.

Russian flags were raised and Russian state broadcasts that promoted the Kremlin's version of the invasion supplanted Ukrainian TV channels.

The Russian rouble this month was introduced as the second official currency in both the Kherson and Zaporizhzhia regions - at least in the parts under Russian control - and pro-Russian administrations started offering a "one-time social payment" of 10,000 roubles (roughly £127) to local residents.

Top Russian officials started touring the regions, touting the territories' prospects for being integrated into Russia.

Deputy prime minister Marat Khusnullin visited Kherson and Zaporizhzhia in mid-May and indicated they could become part of "our Russian family".

A senior official in the Kremlin's ruling United Russia party, Andrei Turchak, put it even more bluntly in a meeting with residents of Kherson: "Russia is here forever."

Members of the pro-Kremlin administrations in both regions soon announced that the areas would seek to be incorporated into Russia.

While it remains unclear when or if it will happen, Russia is laying the groundwork.

An office of Russia's migration services opened in Melitopol, taking applications for Russian citizenship in a fast-track procedure Mr Putin expanded to residents of the Kherson and Zaporizhzhia regions.

The rapid procedure was first implemented in 2019 in the rebel-controlled areas of the Donbas, where more than 700,000 people have received Russian passports.

Oleg Kryuchkov, an official in Russia-annexed Crimea, said this week that the two southern regions have switched to Russian internet providers; state media ran footage of people lining up to get Russian Sim cards for their mobile phones.

Mr Kryuchkov also said that both regions were switching to the Russian country code, +7, from the Ukrainian +380.

Senior Russian legislator Leonid Slutsky, a member of the Russian delegation in stalled peace talks with Ukraine, said that referendums on joining Russia could take place in the Donbas, Kherson and Zaporizhzhia regions as early as July.

Asked about such a scenario, Kremlin spokesman Peskov reiterated on Thursday that it was up to the Ukrainian people to decide their futures but because of the continuing fighting, the conditions were not right for organising annexation referendums.

Tatyana Stanovaya, founder and chief executive of R.Politik, an independent think tank on Russian politics, thinks Mr Putin does not want to rush the referendums and run the risk of them being denounced as shams.

"He wants the referendum to be real, so that the West can see that, indeed, Russia was right, the people want to live with Russia," Ms Stanovaya said.

Ukrainian experts say it is not going to be easy for the Kremlin to rally genuine support in Ukraine's south.

Volodymyr Fesenko, of the Kyiv-based Penta Centre think tank, said residents of the southern regions identify as Ukrainians much more strongly than the people in areas closer to Russia or who have been led by the Moscow-backed separatists for eight years.

"We already see that the occupying Russian administration is forced to tighten the screws and intensify repressions in the southern regions, since it cannot effectively control the protest sentiment," Mr Fesenko said.

"And this causes a new wave of discontent among the population, which received nothing but Russian Sim cards and high Russian prices."

Local residents echoed Mr Fesenko's sentiment.

Petro Kobernyk, 31, an activist with a non-governmental organisation who fled Kherson with his wife, said Russian repression began in the first days of the occupation.

"Hundreds of pro-Ukrainian activists, including my friends, are being held in the basements of security services," Mr Kobernyk said by phone.

"Those who actively express their position are kidnapped and tortured, threatened and forced out of the region."

His claims could not be independently verified.

Russian forces keep people in an "an information vacuum", with Ukrainian websites no longer available, Mr Kobernyk said.

He described a bleak life in Kherson.

With many shops shut down, the city "has turned into an endless market where people exchange goods for medicines and food".